Lithium-ion secondary batteries are known that include a jelly roll made by rolling a positive electrode and a negative electrode, composed mainly of materials that electro-chemically occlude and discharge lithium ions, with an interposed separator in between, or a stack made by stacking a positive electrode and a negative electrode with an interposed separator (both hereinafter referred to as “power generating element”), contained in a metal or laminated casing.
Such a power generating element includes a positive electrode tab and a negative electrode tab connected with the positive electrode and the negative electrode, respectively. If a typical lithium-ion secondary battery has a rectangular or cylindrical metal can for a casing, it generally includes a negative electrode tab and a positive electrode tab welded to the casing can or the lid that seals a casing can or the like. If the casing is made of a laminate of aluminum, for example, one end of each of the positive and negative electrode tabs can be electrically connected to the outside via the laminate, establishing electrical connection with an external terminal to allow electrical power to be exchanged.
In a conventional lithium-ion secondary battery, the power generating element is fabricated in multiple production steps using various assembly apparatus such as a tab welder, a rolling machine, a tab cutter, and other apparatus. Typically, to achieve mass production or for other purposes, the multiple production steps are performed by a single production line, where at least two production lines are concurrently operated to produce the desired number of batteries.
In the context of production management of lithium-ion secondary batteries, improved traceability of batteries is important. Particularly, the safety of a lithium-ion secondary battery often depends largely on the interior structure of the battery, such that it is very important to be able to determine which production line made a battery with a defect in order to deal with an emergency, for example.
Accordingly, methods have been proposed where a number or a figure is impressed on a tab or a current collector as an identifier that achieves traceability (Patent Document 1).